in eager mode.
    *   Callbacks are supported in `MultiWorkerMirroredStrategy`.
    *   Disable `run_eagerly` and distribution strategy if there are symbolic
        tensors added to the model using `add_metric` or `add_loss`.
    *   Loss and gradients should now more reliably be correctly scaled w.r.t.
        the global batch size when using a `tf.distribute.Strategy`.

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